Polylactic acid molding and process for producing the same

Abstract

A polylactic acid formed article and a production process therefor. A resin composition for the formed article comprises polylactic acid having an optical purity of not lower than 90% and a residual lactide amount of 0.1 to 0.6% by mass, and 1 to 25% by mass of a crystal nucleus agent. The formed article is characterized in that a difference (|ΔHm|−|ΔHc|) between the absolute value of a crystal fusion heat amount ΔHm as measured at a heat-up rate of 20° C. / min by means of a differential scanning calorimeter and the absolute value of a heat-up crystallization heat amount ΔHc generated by heat-up crystallization is not lower than 25 J / g, and has a crystallinity of not lower than 35% as determined by X-ray measurement and a crystallization speed of not lower than 0.05 min−1 at 130° C.

Description

TECHNICAL FIELD [0001] The present invention relates to a polylactic acid formed article and a process for producing the same. BACKGROUND ART [0002] With a recent increasing social demand for environmental conservation, biodegradable polymers which are decomposed by microorganism have attracted attention. Specific examples of the biodegradable polymers include fusion-moldable polyesters including aliphatic polyesters such as polybutylene succinate, polycaprolactone and polylactic acid, and aliphatic / aromatic copolymer polyesters such as terephthalic acid / 1,4-butanediol / adipic acid copolymers. Among these aliphatic polyesters, the polylactic acid, which is widely distributed in nature and is harmless to animals, plants and humans, is highly heat-resistant with a melting point of 140 to 175° C. The polylactic acid is promising as a less expensive thermoplastic biodegradable resin. [0003] Where the polylactic acid is molded or formed into a sheet or a container, however, crystals of th...

AI Technical Summary

Benefits of technology

[0008] To solve the aforesaid problems, it is an object of the present invention to provide a polylactic acid formed article which is excellent in heat resistance and shock resistance and can be produced at a higher productivity by a forming process.

[0010] Although the polylactic acid is known as a material having a very low crystallization speed, the crystallinity (crystallization speed) of the polylactic acid per se is increased by limiting the optical purity and residual lactide amount of the polylactic acid as described above. Further, the crystallinity of the polylactic acid after the forming is increased by the addition of a proper amount of the crystal nucleus agent. Thus, the formed article has an excellent heat resistance.

[0019] In the production processes for the polylactic acid formed article according to the first inventive aspect, the resin composition comprising the specified polylactic acid and the specified amount of the crystal nucleus agent is formed into the sheet, and the sheet is heat-treated under the specified conditions prior to or simultaneously with the sheet forming. Thus, the polylactic acid formed article according to the present invention can easily be produced.

[0020] The polylactic acid formed article is advantageously applicable to containers required to have a heat resistance, for example, tableware such lunch trays, bowls, dishes and cups. Since the polylactic acid formed article is free from deformation during storage or transportation thereof in summer, the polylactic acid formed article has a variety of applications for lid materials, building materials, boards, stationery, cases, carrier tapes, cards such as prepaid cards and IC cards, FRPs and various containers. Since the polylactic acid formed article is mainly composed of the biodegradable polylactic acid, the polylactic acid formed article is not accumulated in a natural environment even if being discarded after use. This makes it possible to alleviate an environmental burden to the natural environment and wild animals.

Problems solved by technology

Where the polylactic acid is molded or formed into a sheet or a container, however, crystals of the polylactic acid are generally completely fused due to thermal history experienced during the molding or the forming, so that the resulting molded or formed article is poor in heat resistance.

However, this method is problematic in that the crystallization of the polymer is insufficient without a heat treatment and the productivity is low with a lower crystallization speed of the polymer.

However, the wax employed as the crystal nucleus agent is generally less compatible with the polylactic acid thereby to be bled out.

Therefore, only a small amount of the wax is added, which is insufficient for formation of crystal nuclei.

However, the sheet produced by this method is a stretched sheet, which is further stretched when subjected to a forming process to provide a formed article.

Unfortunately, the sheet once stretched is inferior in drawability and, therefore, is not suitable for deep drawing.

The applications of the sheet are inevitably limited.